The document discusses different forms of energy including work, gravitational potential energy, kinetic energy, and elastic potential energy. It explains that energy cannot be created or destroyed, only changed from one form to another, and provides examples of calculating different energies using formulas like work equals force times distance, gravitational potential energy equals mass times gravity times height, and kinetic energy equals one-half mass times velocity squared. The key idea is that the calculation for one type of energy will yield the same numerical answer as the calculations for the other energy types due to the law of conservation of energy.

1. Work & Energy
In the past…
v, a, x, t  How things move, Kinematics
F, a, m  What makes them move, Dynamics
Now we will look at WHY they move!!! Energy!
Energy  The ability to do work.

2. The theme of this unit is:
Energy is Energy is Energy
Color Code for this power point:
• Energy words will appear in red.
• Forces will appear in yellow.
• Distances will appear in green.

3. Work = Force x Distance
W=Fxd
In units this means
1 Joule = 1 Newton x 1 Meter
Joule (J) is pronounced (jewel) and is the
unit for Work (W) of any kind.
W=Fxd
Newton (N) is the unit for Force (F) and
W=Fxd
Meter is the unit for distance.

4. W=Fxd
For Work (W) to take place on any object
1. There must be a force (F)
2. The object must travel a distance (d).
• Anything that puts a force on an object
displacing it will cause work.

5. Which of the following do work on the box?
gravity No. Doesn’t move up or down
normal force  No. Doesn’t move up or down
you pulling it  Yes, but only the x direction.
friction  Yes, it opposes the motion.
Positive  energy is + if it is going into the object/system
Negative  energy is – if it is coming out of the object/system

6. Example Problem:
How much work will the road do on 18,000 N car
when its brakes are applied, if the car
skids 10 m?
W=F xd
W = 18000 N x 10 m
W = 180,000 J
Which force is at work? Friction because it is
opposing motion.

7. Remember the theme of
this power point?
Energy is Energy is Energy

8. What is that supposed to mean?
Well,
Energy is the
ability to do work
The next slides will tie all energies together.

9. Law of It can only change form.
Conservatio
There are many forms of energy.
n of Energy We will study four types.
 Energy
1.Work
cannot be
2.Gravitational Potential Energy
created or
3.Kinetic Energy
destroyed
4.Elastic Potential Energy
Work: must be a force and distance W = F x d
Gravitational Potential Energy: Energy due to position. Must be a height
off the ground, gravity, and mass.
GPE = m x g x h
Kinetic Energy: Energy due to motion. Must have velocity and mass.
KE = ½ mv2

10. The fourth type is called Elastic Potential Energy
because it is energy due to springs or rubber bands.
We will focus on springs for this lesson.
So SPE means Spring Potential Energy.
SPE requires obviously a spring and how strong it is.
This is represented by little letter k in a formula.
SPE also requires a measure of how far a spring is
stretched or how far it is pushed together.
SPE = (1/2) k x2

11. Conservation of Energy
Now that you know four types of energy and
the theme is
Energy is Energy is Energy
That means
Work = KE= GPE = SPE
F x d = ½(mv2 ) = m x g x h =(1/2) kx2
EACH
TIME YOU CALCULATE ONE THE ANSWER FOR THE OTHERS IS THE
SAME FOR ONE SYSTEM.

12. Example:
A 600 kg roller coaster car is lifted to the top of the
first hill, 55 m above the ground. How much
Potential Energy is this?
Use mass x gravity x height
GPE = 600 x 9.8 x 55
GPE = 323400 J
How much Kinetic Energy does it have as it falls?
323400 J.
How much work did it take to lift it?
323400 J. The answer is always the same.

13. Energy is Energy is Energy
• No matter which one you are asked to
calculate the other energies will have the
same answer in a system.
• This means only calculate the answer one
time and use the answer for all four
energies.

14. • Carefully read over the next slide. This will
illustrate the point about always using the
same answer for calculating energies.
• Remember: Energy cannot be created or
destroyed, it can only change form.

15. Gravitational Potential Kinetic Energy
Energy  energy due to motion
 energy due to position KE = ½ mv 2
PE = mgh
Ex. How much KE does an 1800
Ex. You lift a 1.2 kg book from kg car going 25 mph (11.2 m/s)
the first floor to your social have?
studies class on the 2nd floor 5
m up. How much potential KE = ½ mv2
energy does the book have? = ½(1800)(11.2)2
= 113 000 J
PE =mgh
= (1.2) (9.8) (5) How much work would friction
= 58.8 J need to do to stop it?
W = KE = -113 000 J
How much work did you do? The answer is the same because of conservation of
Conservation!!! energy.
It’s the same = 58.8 J
W = PE answer because of conservation.

16. Sample Problem
A disgruntled physics student drops
her 1.7 kg book off a 4 story building
(12 m), What is the potential energy?
What is the Kinetic Energy? What was
the work done to lift it?
h = 12 m
m = 1.7 kg
PE = m x g x h
PE = 1.7 x 9.8 x 12
PE = 199.92 J
KE = 199.92 J and W = 199.92 J

17. Describe the energy transfer in the following
Different Scenarios
• Dropping an object off a building
• Throwing an object off a building
• Car being slowed down by friction
• You throwing a ball
• A bullet shot; then embedded in a tree
• You lifting your backpack up to math

18. Work Energy Theorem
W = Change in KE
W = Change in PE
W = Change in SPE
In order to change any type of energy,
work must be done.

19. Work Energy Theorem practice
• A 2.5 kg apple hangs from a tree 5 meters up. It falls to
the ground. What is the potential energy of the apple in
the tree?
• What is the final velocity of the apple just before it hits
the ground?
• PE = m x g x h
• 2.5 x 9.8 x 5 Ans. 122.5 J
• KE = ½ mv2
• 122.5 J = ½ x 2.5 x v2 122.5 = 1.25 x v2
• 122.5/1.25 = v2
• 98 = v2 So take the square
root to get v 9.9 m/s = v